Ascaris is a genus of large animal parasitic nematodes. Common species include Ascaris equorum, a horse parasite, Ascaris suum, a pig parasite, and Ascaris lumbricoides, which parasitizes humans. Adult ascarid nematodes, which reside in the host intestine, range from several inches to several feet in length, and females can produce more than 2 million eggs per day. Fertilized eggs are released into the environment by defecation, where they remain dormant until ingested by another host. The acidic environment of the stomach and the increase in temperature trigger resumption of embryonic development, resulting in first-stage (L1) juveniles (often termed larvae) inside the egg shell. After molting once, the larvae hatch from the egg as second-stage (L2) juveniles. Like all nematodes (q.v.), Ascaris molts through three more juvenile stages (L3-L4) and then to a sexually mature adult. Stages in the parasitic life cycle are tissue-specific: The L2 burrows through the wall of the small intestine and is carried via the bloodstream or lymphatic system to the lungs, bronchi, and trachea. It molts to L3 and migrates up the trachea and down the esophagus, returning to the small intestine where it molts to L4 and then, about two to three months after the initial infection, to a mature adult that begins producing eggs. About 25% of the human population is infected by Ascaris lumbricoides, primarily in developing countries. Ascaris infections are generally debilitating but not life-threatening, although heavy infections can cause severe complications resulting from intestinal obstruction, peritonitis, or allergic reaction.
As an experimental organism, Ascaris was important to early embryologists and cytologists such as Theodor Boveri and Otto zur Strassen in the 1890s, who took advantage of several favorable properties of the early embryos that are still exploited by present-day researchers. The embryos are obtainable in large numbers from gravid females; they are transparent, facilitating observation by light microscopy; they can be stored in their original dormant state and their development initiated synchronously by high temperature or acid shock; and the pattern and timing of their embryonic cleavages, like those of most nematodes (q.v.), are invariant from embryo to embryo, with many cell fates apparently determined as the cells are born. The species studied most extensively by Boveri, Ascaris megalocephala, has the additional advantage of having only one large chromosome, which was convenient for cytological studies. Ascaris also has the peculiarity that there is chromosome fragmentation and diminution in all of the somatic precursor cells as they separate from the germ line in the early embryo. Although later work has shown that most of the discarded DNA does not include coding sequences, this feature allowed Boveri to demonstrate that cytoplasmic components specific to the germ line were responsible for maintaining chromosomal integrity in these cells, which supported the view that nonrandomly segregating cytoplasmic determinants were responsible for dictating early cell fates.
In the modern era, Ascaris has also become an important system for research in neurobiology. Part of the rationale behind Sydney Brenner’s choice of the small free-living soil nematode Caenorhabditis elegans (see Caenorhabditis) for genetic and ultrastructural analysis of a simple nervous system and its development was the hope that Ascaris, only distantly related but much more suitable for electrophysiology because of its larger size, would allow functional analysis of a homologous nervous system. This hope has been largely realized: The two nervous systems appear to be very similar in both structure and function, and information from each has been valuable in understanding the other.